Where's the difference between self and $this-> in a PHP class or PHP method?
$this refers to the instance of the class, that is correct. However, there is also something called static state, which is the same for all instances of that class. self:: is the accessor for those attributes and functions.
Also, you cannot normally access an instance member from a static method. Meaning, you cannot do
static function something($x) { $this->that = $x;}because the static method would not know which instance you are referring to.